系统GMM
xtset code year
char province[omit]1
char industry[omit]1
xtdpdsys lnshkl  hhi_labornum subsidyornot /*
*/ d_state d_group d_corporate d_individual d_HK d_foreign  /*
*/ lags(1) maxldep(3) /*
*/ endogenous( lnwr2_city,lag(0,2))  endogenous(age,lag(0,1)) endogenous(size,lag(0,1)) /*
*/ endogenous(KtL,lag(0,1)) twostep vce(robust)